The bolts are frozen, so slotting and using a screw driver is unlikely to work (the socket wrench did not). If you centerpunch the stud remaining(grind a flat face on the stud if possible) and drill dead center down it, and then keep increasing your drill size to the next bolt's drill tap size (consider a metric bolt too) you may re-tap to a reasonable bolt size. Failing this, perhaps you could move all four holes down slightly and drill and re-tap all back into the glassed-in plate. I don't know exactly how much clearance the rudder tiller arms have to the stern etc, but 3/8 of an inch...should work.
The Hobie 16 has those metric cast gudgeons where you re-tap the sterns to suit...that works.